Metallicities and abundance ratios from quasar broad emission lines

The broad emission lines (BELs) of quasars and active galactic nuclei (AGNs) are important diagnostics of the relative abundances and overall metallicity in the gas. Here we present new theoretical predictions for several UV BELs. We focus specifically on the relative nitrogen abundance as a metallicity indicator, based on the expected secondary enrichment of nitrogen at metallicities Z greater than or similar to 0.2 Z.. Among the lines we consider, N III] lambda1750/O III] lambda1664, N V lambda1240/(C IV lambda1549 + O VI lambda1034), and N V/He II lambda1640 are the most robust diagnostics. We argue, in particular, that the average N V BEL is not dominated by scattered Lyalpha photons from a broad absorption-line wind. We then compare our calculated line ratios with observations from the literature. The results support earlier claims that the gas-phase metallicities near quasars are typically near or several times above the solar value. We conclude that quasar activity is preceded by, or coeval with, an episode of rapid and extensive star formation in the surrounding galactic (or protogalactic) nuclei. Chemical evolution models of these environments suggest that, to reach Z greater than or similar to Z. in well-mixed interstellar gas, the star formation must have begun greater than or similar to10(8) yr before the observed quasar activity.